The production of nitric acid is, on an industrial scale, generally carried out by the Ostwald process by catalytic oxidation of ammonia (NH3) over Pt/Rh catalysts. Here, NH3 is selectively oxidized to nitrogen monoxide (NO) which is then oxidized during the course of the further process to nitrogen dioxide (NO2) and finally reacted with water in an absorption tower to form nitric acid. The Pt/Rh catalysts are configured as thin gauzes clamped on a wide area in a burner. A gas mixture composed of typically about 8-12% by volume of ammonia and air is passed through the gauzes, with a temperature of about 850-950° C. being established at the gauzes due to the exothermic nature of the reaction.
An overview of the procedure for nitric acid production and its various process variants is given, for example, in Ullmans Encyclopedia of Industrial Chemistry, Vol. A 17, VCH Weinheim (1991) or in Winnacker-Küchler, Chemische Technik, Prozesse and Produkte, 5th edition, volume 3, Anorganische Grundstoffe, Zwischenprodukte, Chemische Technik, Dittmeyer, R./Keim, W./Kreysa, G./Oberholz, A. (editors), Wiley-VCH, Weinheim, (2005).
Unfortunately, however, the oxidation of NH3 to NO is not 100% selective but a certain proportion of nitrogen (N2) and nitrous oxide (N2O) is always also formed in addition to the desired NO.
Depending on the oxidation conditions, i.e. prevailing pressure, temperature and inflow velocity to the NH3 combustion and also type and state of ageing of the Pt/Rh gauze catalysts, about 4-15 kg of N2O are typically formed per metric ton of HNO3. This results in typical N2O concentrations of from about 500 to 2000 ppmv in the process gas.
The N2O formed is not absorbed when the process gas is fed into the absorption tower and thus goes into the tailgas of HNO3 production. Since the deNOx stages installed here for reducing the residual content of NO and NO2 (together referred to as NOx) also generally do not bring about a reduction in the N2O content, the N2O finally goes more or less undiminished into the atmosphere. For example, the tailgas from a nitric acid plant in which the oxidation of NH3 is carried out at intermediate pressure (about 4-5 bar abs) contains on average about 1000 ppmv of N2O, which corresponds to an N2O concentration in the process gas downstream of the NH3 oxidation of about 830 ppmv.
While NO and NO2 have long been known as compounds having ecotoxic relevance (acid rain, smog formation) and limit values for NOx emissions and technical measures for reducing their amounts have become established worldwide, nitrous oxide has become a focus of environmental concern only in the last decade since it contributes to a not inconsiderable extent to the degradation of stratospheric ozone and to the greenhouse effect. A variety of solutions for removing N2O, partly in combination with new processes for NOx reduction have therefore been developed in recent years for the nitric acid process and employed in industrial plants for the production of nitric acid.
An overview of various measures for reducing the amounts of N2O and NOx in the HNO3 process is given, for example, in: J. Perez-Ramirez et al., “Formation and control of N2O in nitric acid production—Where do we stand today?” Appl. Catal. B Environmental 2003, 44 (2), 117-151, in M. Schwefer, R. Maurer, M. Groves, “Reduction of Nitrous Oxide Emissions from Nitric Acid Plants” Nitrogen 2000 International Conference, Vienna, Austria, March 2000, or in Integrated Pollution Prevention and Control Reference Document on Best Available Techniques for the Manufacture of Large Volume Inorganic Chemicals—Ammonia, Acids and Fertilisers, European Commission August 2007.
For the removal of N2O alone, secondary measures which are directed at decomposition of N2O in the process gas of HNO3 production are frequently used. Here, specific catalysts are installed directly downstream of the NH3 combustion underneath the Pt/Rh gauze catalysts. The process gas here has temperatures of about 900° C., so that N2O here requires only a little catalytic activation to decompose it. The aim of a secondary measure is to achieve very high degrees of removal of N2O. An N2O removal of >80%, often even >90%, is typically achieved. At an average amount of N2O formed of 830 ppmv, which is typical, i.e. average, for an intermediate pressure plant; this corresponds to residual N2O concentrations of <165 ppmv, in particular <80 ppmv, in the process gas or <200 ppmv, in particular <100 ppmv, in the tailgas of HNO3 production. However, degrees of removal of >95% cannot be achieved by means of this technology since the space available for accommodating the secondary catalyst underneath the Pt/Rh gauze catalysts is limited.
However, the secondary measure offers the advantage of universal applicability, usually simple installation and a small catalyst requirement. In the ideal case, only replacement of packing elements which are often arranged underneath the gauze packings for flow equalization by the secondary catalyst is necessary, so that no additional apparatus costs are incurred. Particularly in the case of retrofitting, this is a clear advantage over N2O removal from the tailgas of HNO3 production (known as tertiary measure).
A disadvantage of secondary measures is, however, that owing to the limited space underneath the catalyst gauzes, a correspondingly finely divided catalyst having a high geometric surface area has to be used in order to achieve high degrees of removal of N2O. This is associated with a correspondingly high pressure drop, which is ultimately reflected in a reduced production output of the HNO3 plant. In addition, there is the risk that an only imprecisely definable loss of product can occur since the catalyst can, at 900° C., decompose not only N2O but also NO to an unknown extent.
To remove NOx from the tailgas of HNO3 production, classical SCR catalysts based on TiO2/V2O5 are usually employed in nitric acid plants (cf., for example, G. Ertl, H. Knözinger, J. Weitkamp: Handbook of Heterogeneous Catalysis, vol. 4, pages 1633-1668, VCH Weinheim (1997)). These operate in a temperature range from about 150 to 450° C. and on an industrial scale are preferably operated in the range from 200 to 400° C., in particular from 250 to 350° C. With appropriate dimensioning of the catalyst beds, removal of NOx down to residual concentrations of 40 ppm of NOx, in special cases down to 20 ppm of NOx, can be achieved in this way. In many nitric acid plants, such SCR catalysts are operated in the tailgas in combination with a secondary measure, i.e. together with N2O removal in the process gas.
With regard to NOx removal in the tailgas from HNO3 production, iron-loaded zeolite catalysts also appear to be particularly advantageous since these also enable, unlike classical deNOx catalysts based on TiO2/V2O5, a certain proportion of N2O to be removed at the same time, depending on the temperature. This is, for example, known from the disclosures in DE 101 12 444 A1 and in DE 102 15 605 A. In DE 101 12 444 A1, a gas containing N2O and NOx is firstly mixed with a gaseous reducing agent for NOx, preferably with NH3, and subsequently passed over the catalyst at a space velocity to be selected over the catalyst for the simultaneous removal of N2O (by decomposition) and NOx (by reduction) at a temperature of less than 450° C. In DE 102 15 605 A, the gas containing N2O and NOx is firstly mixed with ammonia as reducing agent for NOx and additionally with hydrocarbons or carbon monoxide and/or hydrogen as reducing agent for N2O and subsequently passed over iron-loaded zeolites for the removal of N2O and NOx, in each case by reduction, at a temperature of less than 450° C. A prerequisite for effective reduction of the N2O in this process is complete reduction of NOx. The removal of N2O in the tailgas from HNO3 production is referred to as tertiary measure.
Various possible ways of avoiding N2O and NOx emissions in nitric acid plants have thus been known to those skilled in the art from the prior art. Here, the abovementioned secondary and tertiary measures for removal of N2O are competing technologies. A combination of these measures for removal of N2O has hitherto not been realized on an industrial scale for cost reasons. In “Remarks and Comments on Nitric Acid Production Project Protocol—Public Draft Version 1.0 October 2009 (obtainable via http://www.climateactionreserve-.org/wp-content/uploads/2009/06/NAP_Public_Comment_-_Uhde_GmbH.pdf) by Groves and Rieck, it is merely mentioned that a secondary measure having poor removal performance could be supported by a tertiary measure in order then to achieve an overall high degree of removal of N2O. It is not stated how the coupling of these measures should be configured, for example whether the tertiary measure is a catalytic decomposition or reduction of N2O or whether the removal of N2O could be coupled with a deNOx stage or which devices or apparatuses could advantageously be used.